The second questions is easy: PostgreSQL won't pull the fields from the table until it really needs them. If a query involving an ORDER BY along with LIMIT and OFFSET is optimized to use the index for the ORDER BY part, the table lookups won't happen for the records skipped.

Though PostgreSQL does not reflect the table lookups in the EXPLAIN output, a simple test would show us that they are done only LIMIT times, not OFFSET + LIMIT (like MySQL does).

Now, let's try to answer the first question: will this trick improve the queries against an InnoDB table?